Checking a border of bedrock or a thickness of a sedimentary layer in a riverbed of a river or lake is essential to calculating a sediment yield for riverbed construction, ensuring the stability of bridge design, and optimizing excavation for canal design.
In order to check a border of bedrock, drilling is generally used. The drilling may give most exact data for borders of bedrock, sediment and weathered layer, but this method just gives depth information limited to the drilling spot and takes much time and cost for the drilling work.
As another method, GRP (Ground Penetrating Radar) survey or shallow survey may be applied, but they have the following problems. First, the GRP survey has a limitation in the depth of investigation according to electric conductivity of a river or lake, and particularly penetration of radar wave is limited in case a soil sedimentary layer has a low electric conductivity. Also, the shallow survey needs development of a suitable transmitting source, so it is also required to develop a receiver sensor accordingly.
Due to the above problems, the present invention is directed to applying an electric resistivity survey at water-covered area. Generally, the electric resistivity survey is conducted in a way of flowing a certain current to a conductor and then calculating a potential difference between predetermined two points to calculate resistivity between two points. That is to say, the electric resistivity survey is a geophysical method for intentionally flowing current into the ground to measure a potential difference and then calculating electric property distribution of an underground media to check a geological structure or anomaly, thereby imaging a geological structure that is complicatedly formed due to irregular media.